38 G. F. MATTHEW: ILLUSTEATIONS OF 



Tlie young- individual figured differs from the adult, as shown by several fuU-growu 

 shells, in having a wider marginal area and a narrower central scar ; the little pits at the 

 back of this scar are also fainter, and the (muscular ?) elevation behind has a larger surface, 

 though less prominent in the young than in the adult. As compared with the young 

 individual, the adult shell shows a condensation of the parts in the posterior segment of 

 the valve, and a shortening of the space between the umbo and the posterior margin. The 

 adult shell is also proportionately wider than that of the young. 



Ventral valve depressed-conical. Umbo about one third from the posterior margin, 

 terminating in a round foramen, wliich is encircled by a narrow elevated rim. The surface 

 of the valve slopes rapidly from the umbo to the posterior margin, or is slightly concave, 

 but in front of the umbo, the slope to the anterior margin is a little convex. The margin 

 itself is reflexed all around, especially in the anterior and lateral part. In many examples of 

 the shell there is }io trace of an area, but the lines of growth appear to go regularly round 

 the posterior slope; in a few. a faint furrow, sloping broadly on each side, to the posterior 

 margin of the axial line breaks the uniformity of the encircling lines, and gives the 

 appearance of a false area, but this appearance may be due to exfoliation of the shell, 

 revealing an internal ridge. Usually there is no foraminal groove visible, but some exam- 

 ples of the ventral valve have a narrow, transversely striated depression, extending on the 

 axial line from the umbo to the posterior margin. 



The mould of the interior of a ventral valve of this species, rather than of any other 

 known at this horizon in the St. John group, has, in front of the foramen, a large, 

 rounded, ovate, median lobe, enclosed by a flattened, horseshoe-shaped area, extending 

 to the margin of the valve on each side of the median lobe. The remaining (posterior) 

 part of the valve has a muscular impression on each side of the axial line behind the 

 foramen, from which, on each side of these scars, an obscurely defined ridge extends to the 

 margin of the valve ; the foraminal opening is encircled by a raised ridge from which a 

 faint ridge extends along the axial line to the posterior margin, The flattened lateral 

 slopes of the valve are bounded on the inner side by a furrow, which is faint along the 

 posterior end of the slope, more clearly defined opposite the foramen, and suddenly deepens 

 where it turns forward along the side of the median lobe : on each side of the A'alve, at 

 the posterior end of the lateral slopes, two faint depressions arch forward across these 

 slopes, from the foraminal side of the slope toward the lateral margin of the valve ; a sig- 

 moid row of minute pits, whose general course is paralled to the axial line of the A^alve 

 extends from the posterior toward the anterior ends of the lateral slopes of the valve ; a 

 fainter row of minute pits arches round the anterior end of the median lobe. Both the 

 lateral slopes and the median lobe are covered with diverging, forking, raised lines, which, 

 on the back and middle of each of the lateral slopes, are trausA^erse, biit on the median lobe 

 are directed forward : on the sides of the median lobe, these lines are nearly parallel to the 

 furrow which bounds it, but on the middle and front diverge and fork in their extension 

 toward the front of the valve. 



Sculpture. The outer surface in both valves is covered with closely-placed concentric 

 striœ, and there are more strongly marked concentric lines of growth, usually three or four, 

 between the umbo and the margin. There, are also finer, radiating ridges, diverging from 

 the summit of the valves to the margin ; these with the lines of growth, often found crowded 

 along th(^ margin, give that part of the valves a cancellated appearance. There is a minute 



